The present invention relates to the manufacturing of semiconductor devices, and more particularly, to a method of using no-clean flux during the attaching of electronic components to substrates.
Interconnection and packaging related issues are among the main factors that determine not only the number of circuits that can be integrated on an electronic computer chip (xe2x80x9cchipxe2x80x9d), but also the performance of the chip. These issues have gained in importance as advances in integrated circuit chip design have led to reduced feature sizes of transistors and enlarged chip dimensions. The semiconductor industry has come to realize that merely having a fast chip will not result in a fast system; it must also be supported by an equally fast and reliable package.
An increasingly important aspect of manufacturing an integrated circuit chip, also referred to as an integrated circuit die or semiconductor die, is the mounting of the die to a substrate to form a package. Essentially, the package, or packaging, supplies the chip with signals and power, and performs other functions such as heat removal, physical support and protection from the environment. Often times, the goal of this process is to provide the chip with as many input/output (xe2x80x9cI/Oxe2x80x9d) terminals as possible. Another important function is simply to redistribute the tightly packed I/Os off the chip to the I/Os of a printed wiring board.
An example of a package-chip system is the xe2x80x9cflip-chipxe2x80x9d integrated circuit mounted on an area array organic package. Flip-chip mounting entails placing solder bumps on a die or chip, flipping the chip over, aligning the chip with the contact pads on a substrate, and re-flowing the solder balls in a furnace to establish bonding between the chip and the substrate. This method is advantageous in certain applications because the contact pads are distributed over the entire chip surface rather than being confined to the periphery as in wire bonding and most tape-automated bonding (TAB) techniques. As a result, the maximum number of I/O and power/ground terminals available can be increased, and signal and power/ground interconnections can be more efficiently routed on the chips.
In the flip-chip bonding process, the die is mounted directly to the substrate. A representation of a flip-chip 10 is illustrated in FIG. 1. Generally, the flip-chip process entails forming regions of solder, e.g. solder bumps 12, on contact pads on the circuit-bearing upper surface 14 of the die 16. Such solder regions may also be formed on corresponding bonding pads 18 on the substrate 20. Flux is then applied to the solder regions on the die 16 and/or to the corresponding bonding pads 18 and/or corresponding solder regions on the substrate 20. Thereafter, the die 16 is flipped and the circuit-bearing upper surface 14 of the die 16 is brought to face the substrate 20. The solder bumps 12 on the die 16 are then brought into contact with the corresponding bonding pads 18. The resulting assembly of the die 16 and substrate 20 is then heated to melt and reflow the solder bumps 12 on the die 16. Upon cooling and re-solidification, each solder bump 12 forms a solder connection between the die 16 and the substrate 20, with the solder joint functioning as both an electrical and a physical connection. Also, the resulting solder joints between the die 16 and substrate 20 are typically encapsulated in an encapsulant 24, also known as underfilling.
The solderjoints are made between solderable metallized surfaces, such as Cu,Cu plated with Pb-Sn, Ni, Ni plated with Au, and with lead solder or other solders containing Sb, Sn or Bi. Because the metallized surfaces to be bonded are typically heavily contaminated with metal oxides, carbon compounds, and other materials due to extended exposure in the manufacturing environment, the surfaces, therefore, require cleaning prior to bonding as a metallized surface contaminated by these materials is difficult to be wetted by solder. However, once this surface contamination is removed, the solder can wet the metallized surface and form a metallurgically sound solder joint.
Contaminants are typically removed from the metallized surfaces by the application of fluxes. A typical flux consists of active agents dissolved or dispensed in a liquid carrier, such as a flux paste. The carrier for flux is typically alcohol-based, with varying concentrations of acids or salts as activators. The function of the activators is to reduce base metal oxides. The flux has a variety of purposes, which include removing oxides from the metallization; removing oxides on the molten solder to reduce the surface tension and enhance flow; inhibiting subsequent oxidation of the clean metal surfaces during soldering; and assisting in the transfer of heat to the joint during soldering.
Many problems associated with the flip-chip process are generated by use of flux. Depending upon the type of flux, a flux residue remains after reflow welding during which the solder joint is formed. The residue can comprise a carrier, such as rosin or resin that is not evaporated, acid or salt deposits, and the removed oxides. If not removed, this residue can be detrimental to the long-term reliability of an electronic package. The resin can also absorb water and become an ionic conductor, which could result in problems such as electrical shorting, noise generation, and corrosion. Additionally, the residual activator can, over a period of time, corrode the soldered components and cause electrical opens.
The use of fluxes that leave corrosive and/or hygroscopic residues require a process to remove these residues to maintain the reliability of the electronic package. Typical post-soldering cleaning processes use chlorinated fluorocarbons (CFCs), organic solvents, semi-aqueous solutions, or water. However, many of these processes result in emission of CFCs and waste water, which detrimentally add to environmental pollution and production costs. In order to preserve the earth environment, the use of such CFCs has been regulated throughout the world. As an alternative to CFCs, non-regulated cleaning agents have been used. However, such cleaning agents are typically not sufficient to remove flux residue; and therefore, use of these cleaning agents cannot maintain the reliability of the electronic package.
In recent years, commercial interest in xe2x80x9cno-cleanxe2x80x9d and low residue fluxes has significantly increased. The interest has moved from the simple desire to leave no visible residue to the unaided eye, to actually measuring the extent of ionic residue, even if it cannot be seen with the unaided eye. Initially, xe2x80x9cno-cleanxe2x80x9d fluxes did not require post-soldering cleaning with environmentally unfriendly cleaners, as for example CFCs or chlorinated solvents, which rosin-based fluxes normally require. However, such no-clean fluxes can leave a residue that is visible. Thus, if any residue was left, it could be readily removed by rinsing with water or some other environmentally friendly solvent. Most recently, however, cleanliness requirements have increased, and a truly low residue flux, by current standards, leaves little or no measurable ionic or organic residue, even when no residue can be seen with the unaided eye.
Underfilling has been used to solve a problem in flip-chip mounting caused by a mismatch commonly found between the coefficient of thermal expansion of the semiconductor die and that of the substrate. Because of thermal gradients experienced by the semiconductor device during normal operation, the solder bumps which couple the die to the substrate experience significant stresses. These stresses can cause thermal fatigue and connection failures. Underfilling has been commonly used to overcome the thermal mismatch between the die and the substrate. This process involves inserting an encapsulation material, such as epoxy resin or other material, into the space between the semiconductor die and substrate after the die has been soldered to the substrate. In addition to being inserted into the space, surface tension produces a capillary action between the die and the substrate which pulls the epoxy into the space. This encapsulation material surrounds the solder bumps and mechanically couples the die and the substrate, thereby decreasing the stress in the solder joints to improve the lifetime of the semiconductor device.
The use of no-clean fluxes before underfilling, however, can result in voids in the encapsulation material. In accordance with prior art methods, as illustrated in FIG. 2, the distance between the semiconductor die 16 and the substrate 20 is very small, for example 3 mils or smaller, and any significant residue 22 left by the no-clean flux after reflow can block access of the encapsulation material 24 into the space 26 between the semiconductor die 16 and substrate 20. Therefore, although this residue 22 may otherwise be benign, the blocking caused by the residue 22 can cause voids 28 in the encapsulation material. Voids 28 are a problem because any void 28 in the encapsulation material 24 adjacent a solder bump 12 reduces the stress-relieving properties of the encapsulation material 24. Accordingly, a need exists for an improved no-clean flux that reduces or eliminates the residues produced after the reflow process.
Reflow furnaces are used for the reflowing of solder, also known as reflow welding, during the assembly of semiconductor devices by the surface mounting of electrical components to a circuit board or other substrate. With reference to FIG. 1, a typical reflow furnace 30 according to the prior art is illustrated. The furnace 10 includes a conveyor belt 32 for passing the electronic components into a number of different zones 34, 36, 38, 40, which are generally divided into a preheat zone 34, a soak zone 36, a reflow zone 38, and a cooling zone 40.
Many of the packaging used today is ceramic, and a temperature profile during reflow welding for a ceramic package is illustrated in FIG. 4. Upon entering the preheat zone 34, the initial heating of the circuit board begins. The process window for the preheat zone 34 is a 1xc2x0 to 3xc2x0 C./second rise in temperature to between 100xc2x0 C. to 125xc2x0 C. During this time, the solvent in the flux begins to evaporate. In the thermal soak zone 36, the circuit board is raised to 150xc2x0 C. to 250xc2x0 C. in about 60 to 120 seconds. This exposure allows the flux to dry and activate. In the reflow zone 38, the solder is heated to above its melting temperature thereby reflowing to form solder joints. The time during which a solder joint is molten is approximately 60 to 120 seconds, and the peak temperature of the leads in the solder joints is typically 360xc2x0 C.xc2x120xc2x0 C. Upon reaching the cooling zone 40, the circuit board is cooled at a rate not more than 4xc2x0 C./second, during which time the solder joints solidify. These profile times and temperatures can vary depending upon the type of semiconductor device, circuit board size, board density, throughput requirements, type of reflow equipment, and solder paste.
The current trend, however, in integrated circuit chip packaging technology is to shift from thick ceramic packaging to relatively thinner organic substrate-based packaging for single chip modules (SCMs) and multi-chip modules (MCMs). However, organic packaging has a higher sensitivity to elevated heat profiles. Because of this sensitivity to elevated heat, the temperature profile during reflow welding for an organic package is different that the temperature profile during reflow welding for a ceramic package. This different temperature profile for organic packages causes problems with the use of no-clean fluxes.
No-clean fluxes depend upon high temperatures to volatilize their constitutes to thereby leave little or no residue. As such, when a given no-clean flux is exposed to a reduced temperature profile, all of the constituents within the flux cannot completely volatilize. The constituents that remain are residue, which has the detrimental affects previously discussed. Accordingly, a need exists for an improved flux for use with organic packages that leaves little or no residue and is active at low temperatures.
This and other needs are met by embodiments of the present invention which provide a method of attaching a die to a substrate using solder to form a semiconductor device. This method comprises applying flux to the die or the substrate; heating the die and the substrate to bond the die to the substrate, cooling the die and the substrate, and underfilling between the die and the substrate. The step of heating the flux includes controlling oxygen and moisture content of an atmosphere surrounding the flux, preheating to a temperature of about 145xc2x0 C. to about 165xc2x0 C., soaking at a temperature of about 145xc2x0 C. to about 165xc2x0 C. for about four to about six minutes, and reflowing above the solder""s melting point.
In further embodiments of the invention, the flux is a no-clean flux, the substrate is an organic substrate, and the die is a flip-chip. Additionally, the temperature increase during the preheating is at a rate no greater than 50xc2x0 C./minute. Also, the temperature increase during the reflowing is at a rate between about 30xc2x0 C./minute about 60xc2x0 C./minute. During the cooling phase, the die cooled at a rate not more than 50xc2x0 C./minute. The solder is typically molten for about one minute to about two minutes, and the peak temperature during reflowing is between about 200xc2x0 C. and about 270xc2x0 C.
By having a soaking phase at temperatures from about 145xc2x0 C. to about 165xc2x0 C. for about four to six minutes, the present invention allows no-clean fluxes to be used with organic packaging. Furthermore, use of a no-clean flux that forms little residue negates the need for an additional cleaning step after reflow and prevents the formation of voids during any subsequent underfilling operation.